REORG: include: create new file haproxy/api.h

REORG: include: create new file haproxy/api.h

This file includes everything that must be guaranteed to be available to
any buildable file in the project (including the contrib/ subdirs). For
now it includes <haproxy/api-t.h> so that standard integer types and
compiler macros are known, <common/initcall.h> to ease dynamic registration
of init functions, and <common/tools.h> for a few MIN/MAX macros.

version.h should probably also be added, though at the moment it doesn't
bring a great value.

All files which currently include the ones above should now switch to
haproxy/api.h or haproxy/api-t.h instead. This should also reduce build
time by having a single guard for several files at once.